Many aspects of mammalian physiology and behavior exhibit a daily 24 hour rhythm. These daily oscillations are circadian rhythms and are controlled by a brain structure known as the suprachiasmatic nucleus (SCN). The mammalian circadian clock is constantly being reset by the onset of environmental light. Light entrainment of the clock requires input from the retina, which communicates with the SCN via the axonal projections of a small subset of retinal ganglion cells (RGCs). Surprisingly, rod and cone photoreceptors are not required; instead, RGCs that project to the SCN appear to function as autonomous circadian photoreceptors as they exhibit light responses independent of rod- and cone-driven synaptic input. These SCN-projecting RGCs also express melanopsin, a novel vertebrate opsin, which is necessary for initiating the light response in these cells. Among all known vertebrate opsins, melanopsin is unique; it shows greater sequence similarity to invertebrate rhabdomeric photoreceptor opsins than to other vertebrate opsins. The current understanding of melanopsin's biochemical properties is rudimentary and its in vivo second messenger system has yet to be conclusively established and extensively characterized. The overall goal of the proposed research is to characterize the spectral and biochemical properties of melanopsin and to determine the biochemical pathway that mediates the intrinsic light responses of SCN-projecting RGCs. We hypothesize that melanopsin forms a photopigment that has biochemical characteristics similar to visual pigments associated with rhabdomeric photoreceptors and activates a Gq-based signaling pathway. Furthermore, we hypothesize that melanopsin undergoes light-dependent modifications that contribute to light adaptation of the melanopsin-dependent signaling cascade. To test these hypotheses, we propose an interdisciplinary approach combining biochemistry, electrophysiology, molecular genetics and behavioral studies. This approach is designed to determine melanopsin's photochemistry, to elucidate the nature of the second messenger pathway activated by melanopsin, and to determine if there are light-dependent post-translational modifications of melanopsin. PUBLIC HEALTH RELEVANCE: Many aspects of mammalian physiology and behavior exhibit a daily 24 hour rhythm. These daily oscillations are circadian rhythms and are controlled by a brain structure known as the suprachiasmatic nucleus. The mammalian circadian clock is constantly being reset by the onset of environmental light. Light entrainment of the clock requires input from the retina, which communicates with the suprachiasmatic nucleus via the axonal projections of a small subset of retinal ganglion cells. Surprisingly, classical photoreceptors, rods and cones are not necessary; instead, suprachiasmatic nucleus - projecting retinal ganglion cells function as autonomous photoreceptors. These ganglion cells also express a novel vertebrate visual pigment, known as melanopsin, which is necessary for initiating the light response in these cells. The proposed research project aims to characterize melanopsin. Understanding the signaling cascade activated by melanopsin is of great interest and significance for the field of sensory biology and circadian rhythms. In addition, disorders of the circadian system often accompany neurodegenerative diseases, sleep disorders, and blindness, and are more commonly experienced as a result of transmeridian travel and shift work. In the future, elucidation of the melanopsin-based signaling cascade should allow us to develop successful pharmacological treatments for these disorders.